Method and apparatus for the generation of low frequency sound

ABSTRACT

The invention relates to a method and apparatus for the generation of low frequency sound by means of the excitation of a low frequency standing gas borne sound wave. A low frequency sound generator comprises a resonator part and a feeder part with a feeder unit. The method consists of a controlled generation of periodic changes of the gas volume of the resonator part in order to create a standing sound wave. The invention also includes a feeder unit for the working of the method. The feeder unit comprised of three rotating parts, namely one center rotor and two side rotors, the rotation of which are synchronized. The feeder unit is open on two sides, each side communicating with one tubular resonator. The rotors are provided with cut out portions and the center rotor is also provided with a protruding vane-shaped part. It is the rotation of the rotors in combination with their special shapes that generate periodic changes of the gas volume in the resonators without any internal compression in the feeder unit.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and an apparatus for the generation oflow frequency sound. In particular the apparatus according to theinvention relates to a feeder unit, also called exigator or pulsator, tobe used with a low frequency sound generator. In addition to the feederunit a low frequency sound generator includes some type of resonator,and the purpose of the feeder unit is to excite a low frequencystanding, gas bourne, soundwave inside the resonator. By low frequencysound is, for the purpose of this context, understood sound of afrequency of less than 60 Hz and even infrasound of less than 20 Hz.

The low frequency sound generators are typically used to excite astanding gas-bourne sound wave and it is the resulting oscillatingmovement of the gas which may be utilized for industrial purposes.Various types of low frequency sound generators for industrial use arepreviously known, for example through EP, B1, 0 006 833 and WO 88/07894.However, for some applications of low frequency sound, for example asdescribed in WO 90/05275, it is desirable to use more than one lowfrequency sound generator. The operation of these low frequency soundgenerators must then be coordinated and synchronized in order to getmaximum effect. As described in the mentioned WO 90/05275 this may beachieved by means of letting two motor driven feeder units be driven bya common motor. However, the apparatus according to the invention hereindescribed offers a simpler solution to the problem of synchronization.Instead of using two feeder units, the apparatus herein describedprovides the possibility of employing only one feeder unit and thisfeeder unit is capable of servicing two resonators.

The present feeder unit has been designed in such a way that it may alsobe used to service only one resonator.

The basic principle for the method and the operation and design of thefeeder unit (or pulsator) according to the invention is to generateperiodic changes of the volume of a resonator and by having the periodof the volume changes correspond to the natural frequency of theresonator a standing sound wave is excited in the resonator. The feederunit comprises of three rotating parts, rotors, of basically cylindricalshapes and which are mounted in a casing. The rotors are driven by acommon motor and their rotation is subsequently automaticallysynchronized. On two sides of the rotor complex a resonator is mounted.The rotation of the rotors in combination with their very special shape,together result in that a small volume of air or other gas is beingtransported from one resonator to the other resonator and then back tothe first resonator. This back and forth transport of air is acontinuous process in that as long as the rotors are rotating air isbeing transported from either of the resonators to the other resonator.By feeding a volume of air into either resonator and then by means ofthe movement of the rotors first decrease the volume of the resonatorand then increase said volume and repeating this sequence with a certainperiodicity, a standing, gas bourne, sound wave of a certain lowfrequency is excited inside the resonator corresponding to the firstnatural frequency of the resonator, which is determined by itsdimensions. This is achieved by having the rotation frequency of therotors, which is determined by the motor, correspond to the naturalfrequency of the resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall now be described in more detail with reference madeto the accompanying drawings illustrating embodiments:

FIG. 1 shows a sectional top view of one embodiment of the feeder unit;

FIG. 2 illustrates a modified embodiment of the feeder unit;

FIG. 3a-3l are a sequence of schematic drawings representing thedifferent stages in the operation cycle of the feeder unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus according to the invention, as illustrated in theembodiment of FIG. 1, comprises of three rotating bodies 1, 2, 3hereinafter referred to as left side rotor 1, centre rotor 2 and rightside rotor 3, mounted in a casing 4. The shape of all three rotors isbasically a circular cylinder, all three having the same radius and theyare all provided with one or several axial, longitudinal and cylindricalcut out portions 7, 8, 9, 10 on the envelope surface, stretching alongthe entire axial length of the cylinder. The two side rotors are ofidentical shape with one cut out portion 7, 8 of cylindrical shape. Thecentre rotor differs in its shape from the side rotors. The centre rotoris provided with two cut out portions 9, 10 and between the cut outportions a blade or vane-shaped part 11 is protruding beyond thecircumference of the basic cylinder shape of the centre rotor. The blademay be made in one piece with the rotor. All three rotors are mounted bymeans of bearings in the casing 4. Their rotation is synchronized andthey are all being driven by the same motor. The centre rotor rotates inone direction while the side rotors rotates in the opposite direction.This may be achieved through having direct drive of the centre rotor bythe motor while the side rotors are being driven by the same motor usingthe intermediary of a cog wheel or similar arrangement.

The shape of the inside of the casing 4 is determined by the shape ofthe rotors. The inside of the casing circumscribes the major part ofeach side rotor's 1,3 circumference and the side rotors are made to sealagainst the inside of the casing leaving just enough clearance for theside rotors to rotate freely. The centre rotor 2 is situated between theside rotors and in the centre of the casing. Due to the blade 11 of thecentre rotor protruding beyond the circumference of the centre rotor andsince the centre rotor and its blade must be able to rotate in thecasing with just a fine clearance, this means that there is a circularspace in the centre of the casing whose radius is determined by the thelength of the blade of the centre rotor measured from the axis of thecentre rotor. This has the result that a chamber 12, 13 is created oneach side of the centre rotor which is limited by the wall of the casingand the rotors. When rotating, the blade of the centre rotor will sealagainst the inside of the casing or the inner surface of the cut outportions in the side rotors. The envelope surface of the centre rotorwill at certain stages of the operation cycle seal against the envelopesurface of either side rotor. However, whenever any part of the centrerotor seals against a surface there is always enough clearance to allowthe rotors to rotate freely. There are also openings 5, 6 in the casingfor the connection of resonators 14, 15. Thereby the chambers 12, 13 arein constant communication with the resonator on the corresponding sideof the feeder and the air volume in the chambers is in contact with theair volume in the respective resonator. Consequently the feeder isalways open on its two sides towards the resonators.

The embodiment illustrated in FIG. 2 shows an apparatus where the centrerotor 20 differs from the centre rotor of the embodiment in FIG. 1. Thecut out portions are here in the shape of three circular segments cutoff from the rotor. The blade 21 of the rotor is not made in one piecewith the rotor but is a separate part mounted in or on the rotor. Theblade is situated in the centre of one of the cut out portions while thetwo other cut out portions which are identical are located on both sidesof the cut out portion where the blade is mounted. In this embodiment isalso shown how holes may be made in the rotors in order to decreasetheir weight and also to improve their rotating balance which isaffected by the cut out portions.

The apparatus functions as described in the following text wherereference is being made to the sequence of schematic FIGS. 3a-3l andwhich represent different stages in the operation cycle of the feederunit according to the invention. The drawings show a feeder unit withtwo resonators, one on each side. The two side rotors rotates in adirection which is clockwise while the rotation of the centre rotor iscounterclockwise and they all rotate with the same constant speeddetermined by the common motor. Starting with FIG. 3a at the bottom onthe left side and going upwards, FIG. 3a shows a neutral position wherethe sound pressure on both sides of the rotors is zero. In this positionthe contact surfaces, i.e. the envelope surface, of all three rotors are"in contact" with each other. By the expression "in contact" is for thepurposes of this context referring to FIGS. 3a-3l, not meant actual,physical contact. Instead, since there should be a clearance between therotors in this position which should be only enough to allow free motionof the rotors and it is understood that in practice the contact surfacesshall seal against each other so that no air or gas should be able toflow from one of the resonators to the other resonator, the word "incontact" and also "seal" should be understood to mean the abovedescribed condition. In this position the vane or blade of the centrerotor is pointed along the centre line of the opening of one of theresonators, for the purpose of these figures referred to as the upperresonator. In FIG. 3b the rotors have rotated to a position where theblade of the centre rotor has just entered into contact with the casingof the feeder unit and as can be seen there is already an openingcreated between the left side rotor and the centre rotor. This showsthat no internal compression takes place inside the feeder unit. Insteadthe air volume between the left rotor, the casing and the centre rotoris in contact with the air volume in the lower resonator. It is thistotal resulting air volume enclosed between the rotors and inside theresonator which is subject to the volume changes caused by the movementof the rotors, primarily the blade of the centre rotor, as shown inFIGS. 3c, 3d and 3e. At the same time the air volume of the upperresonator including the air volume in the upper part of the feeder unitexpands thus resulting in lower air pressure on the upper side of therotors.

In the following FIG. 3f the blade is just about to cease its contactwith the casing and there is still a small opening between the centrerotor and the left side rotor. In accordance with FIG. 3b, no internalcompression will have taken place at the moment when the blade leavesthe casing and thereby lets the air volume between the mentioned rotorsand the casing come into contact with the air volume in the lowerresonator thus resulting in one air volume.

In an analogue way FIGS. 3g-3l show how the air volume in the resonatoron the lower side of the rotors is compressed while the air volume onthe upper resonator side of the rotors is expanded. FIG. 3a shows thesecond neutral position of the apparatus.

By repeating the cycle described in FIGS. 3a-3l periodic volume changesare generated in the entire mass of the resulting air volume on eitherside of the rotors and when these volume changes are controlled, bymeans of controlling the rotation speed of the rotors, to have a periodcorresponding to the natural frequency of the resonator a standing soundwave is excited.

Another way of describing the operation of the feeder unit according tothe invention is that the synchronized rotation of the rotors achievethat a movement of a certain volume of air or gas is performed from oneresonator to the other resonator and that this movement is repeated inorder to obtain a continuous back and forth movement of that volume ofair or gas. Or, while air or gas is being evacuated from one resonator,the corresponding volume of air or gas is simultaneously being fed tothe other resonator and vice versa.

The most common type of resonator to be used would be a tubularresonator. A suitable length of the resonator would be a lengthcorresponding to a quarter or half of the wavelength of the lowfrequency sound generated. When two tubular resonators are connected tothe openings 5, 6 of the feeder unit the standing sound waves excitedinside the resonators will be in counter-phase with each other. If thetwo tubular resonators are of the quarter-wavelength type with one openend they may be connected at their open ends. By doing this oneresonator of the half-wavelength type is created having the same naturalfrequency as the separate quarter-wavelength resonators, and a commonstanding sound wave will be the result. However, depending on the spaceavailable and the surroundings where the feeder and resonator are to beutilized, other types of resonators may be used.

The feeder unit according to the invention is not limited to use withtwo resonators but may also be used with only one resonator. This may bedone by simply installing a cover over one of the openings or a smallbox of some kind.

In the description the resonators and feeder unit have described tocontain air but naturally any other suitable gas may be used. It wouldalso be obvious that the geometric shapes of the rotors and particularlythe centre rotor are not restricted to what is shown in the enclosedfigures but may be modified within the scope of the invention.

I claim:
 1. Feeder unit for a low frequency sound generator, said lowfrequency sound generator comprising a feeder part including said feederunit and a resonator part for the excitation of a low frequency,standing, gas borne sound wave inside the resonator part, said feederunit being provided with means for controlled generation of periodicchanges of the gas volume inside the resonator part, said meanscomprising a plurality of rotating rotors located in a casing andwherein the rotation of said rotors in combination with their geometricdesigns work to achieve volume changes in the gas inside the resonatorpart.
 2. Feeder unit as claimed in claim 1, wherein said volume changesare achieved by alternatively evacuating gas from and feeding gas to theresonator part.
 3. Feeder unit as claimed in claim 2, comprising threerotors, namely one centre rotor, one left side rotor and one right siderotor, the rotation of the rotors being synchronized and the centrerotor rotating in a direction opposite the direction of rotation of thetwo side rotors.
 4. Feeder unit as claimed in claim 3, wherein saidmeans also comprises of a motor which drives said rotors in order forthe rotation of the rotors to be synchronized and the rotation of therotors is controlled by said motor to generate periodic volume changesin said gas volume of a frequency corresponding to the natural frequencyof the resonator part, thus exciting said standing sound wave.
 5. Feederunit as claimed in claim 4, wherein the rotors are of a basicallycircular cylindrical shape, wherein the two side rotors each have onecurved cut out portion in an envelope surface thereof, wherein thecentre rotor has at least two cut out portions in a envelope surfacethereof, and wherein the centre rotor is provided with a vane-shapedportion protruding beyond the circumference of the basic cylinder shapeof the centre rotor.
 6. Feeder unit as claimed in claim 5, wherein theshape of the cut out portions of the side rotors is such that thevane-shaped portion of the centre rotor, when the rotors are rotating,closely follows the inside of the curved cut out portions of the siderotors as well as the inside of the casing with a fine clearance whichdoes not allow the passage of any air or gas, and wherein rotors haveenvelope surfaces which follows the inside of the casing and each otherwith a fine clearance not allowing any passage of air or gas.
 7. Feederunit as claimed in claim 2, wherein said resonator part comprises of tworesonators, wherein said casing has two openings where the resonatorsare connected, wherein on either side of the rotors a chamber is formedlimited by the inner wall of the casing and the rotors, wherein saidchamber is in constant communication with the interior of the respectiveresonator so that the feeder unit is constantly open on two sides, andwherein it is the combined gas volume of the chamber and the resonatoron the respective sides of the rotors which is subject to the periodicchanges of the gas volume.
 8. Feeder unit as claimed in claim 6, whereinwhen the rotors are rotating and the vane-portion of the centre rotor isnearing one of the openings there is always a gap allowing the passageof air or gas between the centre rotor and the side rotor which isclosest to the vane-portion, said gap not being closed until after thevane-portion has reached the opening, with the result that there is nointernal compression.